With the help of one example, explain the phenomena of co-dominance and multiple allelism in human population

ABO blood group in human being is an example of multiple allelism.
Three alleles for the gene I i.e., ${{I}^{A}}$, ${{I}^{B}}$,i.
When ${{I}^{A}}$, ${{I}^{B}}$ are present together the blood group is AB.
Both A and B are expressed and is called codominance.